SAW devices (surface acoustic wave devices) are installed in communication apparatuses such as cellular phones in order to remove noises included in electric signals. SAW devices have a function of extracting only electric signals having a desired frequency among input electric signals. SAW devices have a structure in which electrodes are formed on a piezoelectric substrate. To dissipate heat during operation, the piezoelectric substrate is disposed on a base substrate made of a material with good heat dissipation properties.
For example, a substrate made of single-crystalline sapphire can be employed as the base substrate. However, if such a substrate made of single-crystalline sapphire is employed as the base substrate, the production cost of SAW devices increases. To address this problem, there has been proposed a SAW device having a structure in which a ceramic substrate made of polycrystalline spinel is employed as a base substrate, and a piezoelectric substrate and a ceramic substrate whose surface roughness Ra (arithmetic mean roughness) is decreased are bonded to each other through Van der Waals force. Thus, the production cost of SAW devices can be reduced (e.g., refer to PTL 1).